1. Field of the Invention
The present invention relates to a process for recovering acetic acid from a mother liquor, and more particularly, to a process for recovering acetic acid from the mother liquor used in the formation of N-formylaspartic anhydride.
2. Description of the Prior Art
Aspartame, .alpha.-L-aspartyl-L-phenylalanine methyl ester, is a sweetening agent which is approximately 100-200 times sweeter than sucrose. As a result, aspartame has been widely used as a substitute for sucrose in beverages, foods, and the like.
A number of methods have been proposed for synthesizing aspartame. One such method comprises reacting N-protected aspartic acid anhydride with L-phenylalanine or its methyl ester, typically in an acetic acid solvent, followed by esterification of the reaction product. This method is described in greater detail in U.S. Pat. Nos. 3,933,871 and 4,810,816. As noted above, the starting material used for the aspartame synthesis comprises an N-protected aspartic acid anhydride, and particularly N-formylaspartic anhydride. The anhydride is typically prepared by reacting amounts of aspartic acid, formic acid and acetic anhydride. This reaction optionally takes place in the presence of co-solvents or metal catalysts. Methods for producing N-formylaspartic anhydride are disclosed in greater detail in U.S. Pat. Nos. 3,933,871; 4,526,985; 4,550,180; and 4,810,816.
The resultant mixture produced by the synthesis of N-formylaspartic anhydride comprises N-formylaspartic anhydride crystals in a mother liquor. In practice, the crystals may be separated from the mother liquor prior to reaction with the L-phenylalanine (or its methyl ester) or the L-phenylalanine may be directly added into the crystals/liquor mixture. The former method is particularly advantageous as it enables higher yields of aspartame to ultimately be obtained and results in a more pure final product as the impurities which are present in the mother liquor are not transferred to the crystals.
The mother liquor remaining after separation of the crystals comprises acetic acid, acetic anhydride, formic acid, formic acetic anhydride and high boiling components derived from aspartic acid, with acetic acid being present in the most predominant amount. Due to the value of acetic acid, it is desirable to recover it in relatively pure form from the mother liquor. One method for doing this comprises separating the aspartic acid derived components from the remainder of the mother liquor and using a four component fractional distillation to isolate the acetic acid. This process is problematical as it is extremely expensive and tedious.
Accordingly, there exists a need in the art for a simplified process for recovering acetic acid from a mother liquor including acetic acid, formic acid, acetic anhydride and formic acetic anhydride.